Transmit/Receive (T/R) switches are used extensively in radio communications electronics. In these applications, the T/R switch is used to couple an antenna to the transmitter and receiver electronics in a manner such that when transmitting, the majority of the transmitter power goes to the antenna and when receiving, the majority of the signal received by the antenna goes to the receiver. The T/R switch also protects the receiver circuitry from being damaged by large transmitter signals through limiting the power that gets to the receiver input section. Several different kinds of electronic circuits have been employed to perform the T/R switch function.
U.S. Pat. No. 4,637,065 to Ruppel discloses one type of T/R switch used in radio communications circuits and contains a good description of the prior art. These types of T/R switch circuits utilize PIN diodes, giving rise to a long switch over time of several milliseconds, which is adequate for most radio communications applications. However, a T/R switch suitable for use with Electromagnetic Acoustic Transducers (EMATs) must be very fast acting; that is, capable of switching from transmit mode to receive mode in a few microseconds or less. Thus, a drawback to using these type of T/R switches for EMAT operation is that the time it takes for the circuit to recover from the transmit mode and then switch to the receive mode is too long.
Another method of coupling the transmitter and receiver to the same transducer is to attach the output of the transmitter directly to the transducer and attach the input to the receiver to the transducer via resistors. As illustrated in FIG. 1, a back-to-back diode (CR11 and CR12) arrangement is placed at the input to the receiver to prevent damage to the receiver circuit from the large transmitter voltages with the current being limited by the resistors. This allows rapid switching from transmit mode to receive mode. However, the use of resistors to couple the transducer to the receiver input results in transmit power loss in the resistors and signal to noise reduction from the receiver because of the increased resistance at the receiver input, so that this arrangement is not well-suited for use with EMATs.
FIG. 2 illustrates another T/R switch circuit known to the inventor prior to the current invention. In this T/R switch, the transmitter output is coupled directly to the transducer. The transducer is then coupled to the receiver input via a power limiting circuit formed by diodes CR1 and CR2, inductor L1, resistor R1, voltage source V1, and transformer T1. Direct current (D.C.), supplied from V1 and limited by resistor R1, flows through the diodes CR1 and CR2. This D.C. is set by R1 such that the diodes are biased "on" for small signals; that is, a small received signal passes from the transducer to the receiver input unimpeded because the diodes are put into a conductive state (low resistance) by the D.C. bias currents. Inductor L1 provides a high impedance for the radio frequency (RF) signals preventing them from flowing through voltage source V1. High voltage RF signals applied to the transducer cause the diodes to become reverse biased, switching them into a nonconductive state. This switching time depends on what diodes are employed in the circuit.
In the circuit illustrated in FIG. 2, the diodes employed had switching times on the order of 1/10 of a microsecond. However, the 1/10 of a microsecond switching time limited the operation to frequencies below approximately 2 MHz because the diodes must switch on and off with each cycle of the transmitter RF toneburst. Consequently, one disadvantage to using this circuit for EMAT operation is that it is single ended (one side of the transducer is connected to ground), and those skilled in the art have found that, in order to prevent noise pickup, the EMAT coil is best kept isolated from ground and operated into a differential input receiver which provide high common mode noise rejection. Likewise, the D.C. bias current flowing through the primary windings of T1 can cause the transformer core to saturate if it is not of adequate size resulting in larger transformer size than would otherwise be necessary. Therefore, the circuit illustrated in FIG. 2 is not well-suited for use with EMATs.
In sum, it is apparent that an improved T/R switch suitable for use with an EMAT is needed to overcome the deficiencies discussed above. Moreover, such an improved T/R switch for use with an EMAT would be welcome by the industry.